cmd/compile: update default ABI choices for calls and bodyless fn stack maps

After recent discussion about bodyless functions, their wrappers,
their stack maps, nosplit, and callbacks, I was inspired to go and
be sure that more defaults were sensible.  This may not be all --
currently rtcall is "ABIDefault" which I think is correct, but I
am not 100% certain.

Updates #40724.

Change-Id: I95b14ee0e5952fa53e7fea9f6f5192358aa24f23
Reviewed-on: https://go-review.googlesource.com/c/go/+/304549


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>